Several methods of measuring the strength distribution of optical fibers have been developed over the years. One known as the dynamic test method relies upon the random selection of a series of lengths, L.sub.0, of fiber from a longer length, L. The fibers are tested to failure in a tensile test machine by applying a constant loading rate .sigma.. The stress .sigma..sub.f at which the fiber fails then is recorded. A plurality of these lengths, L.sub.O, called the gauge length, is measured to provide a statistical sample of size D, usually D is greater than or equal to 30. The data are then ordered in a series ranging from lowest strength to highest and assigned an approximate failure probability F.sub.i (.sigma.) where EQU F.sub.i =i/D+1=1-exp{-(.sigma..sub.fi .sigma..sub..rho. /.sigma..sub.o -.sigma..sub..rho.)}.sup.M ( 1)
In this equation .sigma..sub.fi is the failure stress of the i-th sample, .sigma..sub..rho. is a proof-stress if the fiber was previously proof tested, .sigma..sub.o is a constant and M, the exponent, is constant for a given environment. Graphical plots of the function of equation (1) enable the assignment of an average strength to the fiber length L because the length L.sub.0 is related to the strength of the length L by the relation, S.sub.L =S.sub.Lo exp .theta. (-L/L.sub.0).
Another method of measuring strength distributions is by means of static fatigue testing. In this method the fiber is not subjected to a changing rate of stress but rather a constant loading by a weight W and the time to failure is measured. A variation of this is to wind the length L.sub.0 on mandrels of varying diameters and measure the time-to-failure of breaks in the fiber. The stress of failure is given by the expression EQU .sigma..sub.fs =approx. E(.rho./R.sub.F) (2)
where E is Young's modulus, .rho. the radius of the fiber and R.sub.F is the radius of the mandrel. The relation between time-to-failure under static and dynamic conditions is approximately given by the relationship EQU t.sub.d (.sigma..sub.d.sup.N)=(N+1) (t.sub.s) .sigma..sub.s.sup.N. (3)
where t.sub.d and t.sub.s are the respective dynamic and static times to failure and .sigma..sub.d and .sigma..sub.s are the dynamic and static stresses at failure. The constant N, sometimes called the fatigue parameter, occurs in several known theories of the fracture mechanics of ceramics. It is a measure of the tendency of the material to suffer stress corrosion and relates various fracture mechanic parameters to the velocity of crack propagation preceding from flaws under stress. The disadvantage of these measurement techniques is that they are slow, tedious and require a great deal of handling of the fiber, selecting samples and applying the various techniques for measuring time-to-failure).
David Sinclair of the Johns-Manville Research Center discloses a method for measuring strength in his article entitled "A Bending Method For Measurement of the Tensile Strength and Young's Modulus of Glass Fibers" appearing in the Journal of Applied Physics, volume 21, May 1950, pg. 380 et seq.. Mr. Sinclair discusses a bending method which consists of twisting a loop of fiber, pulling the ends until the loop breaks and measuring the force. While the procedure seemed to produce accurate representations of fiber strength, the number of discrete operations necessary to perform the step-by-step procedure had the appearances of being labor intensive and time consuming. Another technique was disclosed by P. W. France et al in their article entitled "Liquid Nitrogen Strengths of Coated Optical Glass Fibers" in the Journal of Materials Science, 15 (1980), pgs. 325 et seq.. They relied on a man-actuated U-shaped bender for receiving a fiber and stressing it until it breaks for the purposes of determining indications of tensile strength. Here again, the results were found to be satisfactory yet it appeared that a considerable number of manipulations of the fiber and device might make this procedure a bit time consuming and labor intensive.
Thus, there is a continuing need in the state of the art for an automated system for measuring the strength of optical fibers that is consistent to provide acceptable indications of strength yet which does not call for a labor-intensive and time consuming procedure.